1. Field of the Invention
This invention relates to the non-invasive measurement of the concentration of substances that absorb electromagnetic radiation, such as light or infrared radiation, in absorbing and turbid matrices such as human or animal body tissue using a probe beam of electromagnetic radiation. The method and apparatus used to make the measurement according to this invention are fully described in the annexed claims and in the following description accompanied by figures.
The invention is described as applied to the special case of glucose measurement in human tissue using near-infrared radiation. This should in no way detract from the general applicability of the invention to measure the concentration of any species that absorbs electromagnetic radiation, especially in strongly absorbing and turbid matrices.
2. Description of the Prior Art
The infrared measurement methods known in the art are not well adapted to the problem of quantifying an analyte dissolved in a strongly absorbing solvent. The known methods include separate or directly alternating measurements using radiations at a "glucose" wavelength and at a "reference" wavelength, where glucose does not absorb, as well as differential wavelength modulation about a glucose absorption band (C. Dahne, D Gross, European Patent 0 160 768 and references therein). In the known methods, the signal is easily lost into the variable and strong background presented by the water in the tissues and in the capillary blood flow. The reference concentration range of glucose in blood is 2.8-5.5 mmol/l.